A great deal of research has been directed in recent years to developing biodegradable polymer compositions for use in providing for the controlled delivery of biologically active agents, particularly drugs. Such compositions may be implanted into a patient and function to dispense the active agent at a controlled rate sufficient to provide the dosage required for treatment.
Biodegradable polymer compositions release the active agent as the polymer is eroded away by the environment through dissolution hydrolysis processes or enzymatic degradation. When such polymers are used for delivery of drugs within medical applications, it is essential that the polymers themselves be non-toxic and that they degrade into non-toxic degradation products as the polymer is eroded by body fluid.
In order to minimize the toxicity of the intact polymer carrier and its degradation products, polymers have been designed based upon naturally occurring metabolites. The most extensively studied examples of such polymers are the polyesters derived from lactic or glycolic acid [D. L. Wise et al., Drug Carriers in Biology and Medicine, G. Gregoriadis ed., Academic Press, London, 1979, pp. 237-270 and polyamides derived from amino acids D.A. Wood, Int. J. Pharm. 7:1, 1980].
Toxicity of the polymer and its degradation products represents only one of a number of factors which need to be considered in order to provide an effective biodegradable polymer composition for use in the controlled delivery of a drug or other active agent. Thus, the polymer composition must also have suitable physical and mechanical properties including effective surface erosion so that the active agent is released in a controlled manner. Bulk erosion is not satisfactory because this results in a complete breakup of the polymer composition rather than providing a slow or controlled release of the active agent. Bulk erosion usually occurs when the polymer is hydrophilic and absorbs water to the point of becoming sponge-like. Many polymer compositions cannot be effectively used as controlled release biodegradable polymers because they are hydrophilic and undergo bulk erosion. Typical of such polymers are polylactic acid or polyglutamic acid.
Despite the extensive research activity in this field, only a relatively few bioerodible polymer compositions have been developed for in vivo use. Examples of useful compositions are described in U.S. Pat. No. 4,070,347 which discloses polycarbonate and polyorthoester polymeric compositions. Polylactic acid and lactic/glycolic acid copolymers have also been employed for controlled release of biologically active substances. These materials, however, suffer from the problem of bulk erosion referred to earlier.
In recent years a new class of biodegradable polymers, the polyanhydrides, has been introduced for medical use. These polymers display superior physical and mechanical properties with respect to erodible carriers for controlled release drug delivery systems (Rosen, H.B., Chang, J., Wnek, G.E., Linhardt, R.J., and Langer R., Biomaterials, 4 131, 1983; Leong, K.W., Brott, B.C. and Langer R., J. Biomed. Mat. Res., 19, 941, 1985; Mathiowitz, E., Saltzman, W.M., Domb, A.J., Dor Ph., Langer, R., J. Appl. Polym. Sci., 35, 755, 1987). See also Domb, A. J., Ron, E. and Langer, R., Macromolecules, 1988, 21, 1925 and Domb et al, U.S. Pat. Nos. 4,757,128 and 4,789,724.
Notwithstanding the foregoing efforts, there is still considerable room for improvement in biodegradable polymer compositions for use in providing for the controlled release of drugs or the like. Accordingly, the principal object of the invention is to provide such improvements.
A more specific object is to provide biodegradable polymers based on naturally occurring amino acids. An additional object includes the provision of biodegradable polymer compositions based on amino acid derivatives which are useful as carriers for the controlled release of drugs or other active agents. A further object is to provide biodegradable polymers which have improved physical and mechanical properties. Another important object is to provide biodegradable polymers based on bioactive amino acids whereby the polymers themselves or the degradation products thereof can effectively function as the controlled release drug or other active agent. Other objects will also be hereinafter apparent.
Amino acids are the main metabolites of the body. Accordingly, polymers based on amino acids which degrade into their amino acid counterparts offer the possibility of favorable surface and erosion biocompatibility. In addition, polymers with alternating amide, imide, azo, urethane, urea or thiourea bonds and anhydride or ester bonds in the polymer backbone as contemplated herein, have improved physical and mechanical properties as a result of the incorporation of bonds with high cohesion energy.
Another important advantage of forming amino acids into biodegradable polymers is the fact that many amino acids are biologically active and are used as drugs in the clinic. Degradable polymeric drugs, releasing pharmacologically active amino acids in a controlled fashion for extended periods of time, improve therapy effectiveness and bioavailability of the drug (`Design of Prodrugs`, Bundgaard H. editor, Elsevier Sci. Pub., Amsterdam, 1985).
Examples of amino acid drugs suitable for the polymeric drug approach contemplated herein include biologically active natural .alpha.-amino acids, e.g., glycine, .gamma.-amino butyric acid (as brain transmitters); phenylalanine derivatives i.e. L-dopa, D-thyroxine; aminosalicylic acid derivatives; tyrosine derivatives (as adjuvants); .beta.-lactam antibiotics, such as ampicillin and cephalexin, and oligopeptides (as peptidic hormones) with carboxylic acid and amino group terminals, i.e. L-alanyl-DL-alanine, L-alanyl-L-alanyl-L-alanyl-L-alanine, and alanyl-leucyl-alanyl-leucine.